Someone does eventually realize there's something to investigate. Kotetsu notices the scorch marks and charred curtains and follows the trail they make throughout the castle, even if he has no idea who or what is waiting for him at the other end.

He finally rounds the corner and sees Frisk up ahead, and puts on a burst of speed to catch up. "Hey! Hold up a second..."

It's only when he actually catches up that he realizes who he was inadvertantly chasing. The body is different. The face is very much the same.

"Frisk! Wh-What the..." There are a lot of details to zero in on - the lack of a hand is at the top of the list. His gaze snaps to it and a hand flies to his mouth. "Frisk, oh my god. What happened?"